Synchronous rectification is a technique for improving efficiency of power converters in power electronics. It consists of connecting a transistor (typically a power MOSFET) and a diode (typically the body diode of the MOSFET) in parallel. When the diode would otherwise be forward-biased, the switching element is turned on, to reduce the voltage drop. When the diode would otherwise be reverse-biased, the transistor is turned off, so no current can flow through the parallel connection of switching element and diode. This way, a rectifying characteristic is obtained, without the forward voltage drop associated with diodes in the on-state.
The efficiency and effectiveness of synchronous rectification depends on the timing of the switching on and off of the transistor or switch. In particular, it is important that the switch is not turned off too late, lest current flows the wrong way past the rectifier. However, it is also important that the switch is not turned off too early, since this would result in less than ideal rectification, with associated losses. In comparison, for proper functioning, (though not necessarily for efficiency), the switch-on time is generally less critical.
The switching-off should be as close as possible to the zero crossing of the current through the rectifier. For the identification of the zero current state the small voltage drop across the channel of the switching element can be used, as it also can for the detection of the switch-on instant, although the voltage level is considerably lower for switch-off than the level for detection of the on-instant.
So, in order to ensure that reverse current does not flow through the rectifier, threshold voltages can be set to trigger switch-on (at 0.3V, say), and switch-off (typically at a much lower value, for instance at 10-40 mV).
However, the voltage signal seen by the comparator usually includes some parasitic oscillations caused by parasitic components. These include parasitic capacitances and inductances in the switch, the package and the layout. As a result, the signal usually has some superposed oscillations having an approximately fixed frequency which is dependant on the components and the layout. These oscillations will also be referred to hereinafter as ringings. Since the circuit is intentionally made low-resistance in order to minimise ohmic-losses, the ringings are only lightly, or weakly, damped.
FIG. 1 shows the problems arising due to unavoidable oscillations in real applications. The figure shows as (feathered) curve 12 the secondary side current occurring if parasitic components are taken into account. This is a triangular falling current superimposed by a weakly damped high frequency oscillation. The better, or more low-ohmic, the synchronous rectification switch is, the less damped the oscillation becomes. Thus in typical synchronous rectification circuits the oscillation remains as long as the synchronour rectifier switch is closed—which in the case of a flyback corresponds to the secondary stroke.
Curve 13 demonstrates the sensed drain source voltage in case that no parasitic inductance due to the chip and the package is involved. This could be obtained by means of on-chip sensing (Kelvin-contacts). This represents the best case which can be obtained. Already in this case the synchronous rectification detection circuit would switch off the synchronous rectifier much too early since voltage across drain source crosses even the zero level indicated as green line.
Curve 11 depicts the more realistic case that typically occurs if driver and synchronous rectification switch are implemented into two packages. In this case additional package inductances provide additional voltages due to high frequency oscillating currents.
In order to realise the benefit of the synchronous rectification a very accurate detection of the turn-on and especially turn-off time instant is important.
In known synchronous rectifiers, passive filtering is applied in order to reduce the problem of early switch-off. In one such arrangement, leading edge blanking can be used to prevent a detection of the turn-off the signal, until a minimum time period has elapsed. However, if the oscillation is still present and not sufficiently damped the switch will be switched off too early (or, for low power output, too late).
In another arrangement, which is shown schematically in FIG. 2, the comparator level is a shaped over time. Thus, instead of having a fixed threshold level (such as 10-40 mV, the threshold is shaped, to be negative initially and decay exponentially towards a small positive value, as shown at 21. Alternatively, a simple RC filter can be applied to the drain-source switch voltage. However, such passive filtering is readily relatively in-flexible, and there remains a desire for better control of the switch-off moment.